No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 275 



PART II. — COMPARATIVE AND GENERAL. 



In the literature on the embryology of annelids, the record of 

 cell lineage has usually been incidental to the description of the 

 general development of the embryo. Whitman, however, in his 

 well-known work on the embryology of Clepsine, 1878, gave a 

 detailed account of the exact origin of those cells which give 

 rise to the mesoderm, nerve-cords, nephridia, etc. ; he introduced 

 and proved the value of cell lineage as a method of embryological 

 research. 



By far the most important contribution to the cell lineage of 

 annelids since 1878 is that of E. B. Wilson, based on a study 

 of the marine annelid Nereis limbata, 1891. He recorded every 

 cell division up to the 58-cell stage, and many of the subse- 

 quent cleavages. In the appendix to his paper he briefly 

 describes the cleavage of Polymnia, Spio, Aricia, and Eupo- 

 matus. Several investigators have shown that the cleavage in 

 certain fiatworms and molluscs has many features in common 

 with that in annelids ; the most apparent distinguishing pecu- 

 liarities are (a) the obliquity of all the early cleavage furrows, 

 and (b) their regular alternation in direction in successive gen- 

 erations of cells. Cleavage characterized by these features may 

 be designated alternating oblique (spiral type of Lang, Wilson, 

 and others), and has already been described in the following 

 forms. 



Among annelids : 



Clepsine, Whitman®^ ^"^ 



Nereis cultrifera, Salensky^i 



Nereis Dummerillii (free-swimming trochophore), Goette^^ 



Lumbriciis and Rhyjtchelmis, Vejdovsky ^^ 



' Whitman, C. O.: The Embryology of Clepsine. Quart. Jour. Micr. Sci., 

 XVIII, 1878. 



1" Whitman, C. O.: The Germ-Layers in Clepsine. Journ. of Morph., vol. I, 

 1887. 



" Salensky, W.: Etudes sur le Developpement des Annelides. Arch, de 

 Biologic, III, 1882. 



12 Goette, A.: Abhandlungen zur Entwicklungsgeschichte der Tiere. Leipzig, 

 1882. 



12 Vejdovsky, Fr.: Entwickelungsgeschichtliche Untersuchungen. Prag, 1883. 



